Method and device for electrodialytic regeneration of an electroless metal deposition bath

ABSTRACT

A method and a device for the regeneration of an electroless metal deposition bath containing hypophosphite ions by electrodialysis is described. The method according to the invention and the device differ from the prior art in that the bath solution is led simultaneously through diluate compartments in a second electrodialysis unit having cathodes and anodes, which compartments are separated from concentrate compartments in the second electrodialysis unit on the cathode side by monoselective anion-exchange membranes and on the anode side by anion-exchange membranes, the diluate compartments and the concentrate compartments in the second electrodialysis unit being disposed alternately to one another.

This application is a 371 of PCT/DE99/03186 filed Sep. 27, 1999.

The invention relates to a method and a device for regenerating byelectrodialysis an electroless metal deposition bath, especially anelectroless nickel deposition bath.

The electroless metal-plating of workpieces has been known for a longtime. For example, sanitary fittings made of plastics material areprovided with metal layers in order to obtain a specific aestheticappearance, or specific workpieces consisting of metal, in order toimprove their serviceability, for example the wear-resistance orcorrosion behaviour. Thus, in machine-building, parts which aremechanically heavily loaded receive resistant coatings comprising alargely amorphous nickel/phosphorus alloy layer in order to increase theresistance to abrasion, for example of bearing shells on moveable parts.In oil production, metal parts used in the off-shore domain are coatedwith a nickel/phosphorus layer of this type in order to improve thematerial resistance to chemical influences.

Electroless plating with metals is based on an autocatalytic process inwhich dissolved metal ions are reduced to metal by means of a reducingagent located in the deposition solution, and deposited on the workpieceto be coated. In this case, additional components are often incorporatedinto the metal layer, for example phosphorus. As well as nickel, coppercan also be deposited by this method.

For the deposition of nickel/phosphorus layers, electrolytic andelectroless methods can basically be used. Electrolytic methods areadmittedly easier to handle; however they have the disadvantage thatlayers of uniform thickness can only be obtained if the parts to becoated have a simple geometry. The electrolytic metallisation ofworkpieces which have a complex geometry, for example curvatures, holesor undercuts, leads to an uneven layer thickness and thus in many casesto intolerable local fluctuations in the plating result. Moreover, themetal layers deposited in an electroless manner often have moreadvantageous mechanical properties than metal layers deposited byelectrolytic means. For this reason, electroless methods are veryfrequently used for plating.

Electroless metal deposition is represented below in the example ofelectroless nickel deposition with simultaneous incorporation ofphosphorus into the layer. In this process, a deposition solution isused, for example, which contains sodium hypophosphite as the reducingagent for nickel ions, as well as nickel ions, for example as nickelsulphate. The deposition reaction takes place according to the followingreaction equation:

NiSO₄+6 NaH₂PO₂±Ni+2 H₂+2P+4NaH₂PO₃+Na₂SO₄

Thus in this reaction, dissolved nickel and hypophosphite ions areconstantly consumed, whilst the concentration of orthophosphite (H₃PO₃⁻) increases as an oxidation product. Moreover the counterions of thenickel cations and hypophosphite anions accumulate in the form ofNa₂SO₄.

Thus, methods of this type have the disadvantage that the processmanagement is complicated in many cases and a large number of monitoringoperations has to be carried out in order to achieve constant depositionconditions. In addition to this, the service life of the electrolessdeposition baths is limited. In metal deposition, the reducing agent andthe metal ions are used up which have to be continually added as themethod is carried out, in order to make available an approximatelyconstant content of available reducing agent and available metal ionswithin a narrow band width. Since the reducing agent and the saltscontaining the metal ions leave behind, during the deposition reaction,products which accumulate in the deposition bath, the service life ofthe bath is inevitably limited. For example, the metal ions are added tothe bath in the form of salts, such that disturbing anions, such assulphate ions accumulate in the bath. The same is true fororthophosphite ions (H₂PO₃ ⁻) which form in the bath through oxidationof hypophosphite ions.

The age of a bath is generally quoted in metal turnover (MTO). 1 MTOcorresponds to the amount of deposited metal from the bath whichcorresponds to the initially used concentration of the metal ions in thebath, respectively in relation to the total volume of the bath.Generally, the degradation products in the bath reach such a highconcentration after 6 to 10 MTO that the quality and deposition speed ofthe metal are no longer within tolerable ranges. Therefore baths of suchan age are not used again. A new bath must be started and the spent onemust be thrown away. What is disadvantageous is that the necessarydisposal of the baths and the required new charging of fresh baths leadto high costs and considerable damage to the environment. For thisreason, different methods have been proposed by means of which theservice life of baths of this type can be extended.

In U.S. Pat. No. 5,221,328, a method for extending the service life ofelectroless nickel baths is described, by means of which methodorthophosphite which has been produced in a nickel/phosphorus depositionbath can be precipitated as a metal salt and separated. Yttrium andlanthanides can be considered as precipitants. However, the necessarychemicals for this are extremely expensive. Moreover, the dissolvedcomponents of these additives, remaining in the bath, can impair thequality of the metal coatings.

In “Plating and Surface Finishing”, September 1995, pages 77 to 82, itis proposed by C. D. Iacovangelo that the disturbing precipitation ofnickel orthophosphite be prevented through the addition of complexingagents. By this means, the concentration of dissolved free nickel ionsis reduced.

In the ENVIRO CP-process of the company Martin Marietta, U.S.A., thedisturbing components in the bath are separated by means of adsorptionon ion-exchange resins. For the complete separation and regeneration ofthe deposition bath, a complex process is carried out in which aplurality of different ion-exchange columns and containers for diverseprocess liquids are needed.

Y. Kuboi and R. Takeshita describe a method of separating the undesiredbath components by electrodialysis (Electroless Nickel Conference 1989,Proceedings, Prod. Finishing Magazine, 1989, pages 16-1 to 16-15). Inthis method, the electroless nickel bath is led as so-called diluatethrough an electrodialysis cell. The diluate compartment in theelectrodialysis cell is, for this purpose, separated on the anode sideby an anion-exchange membrane from the anode compartment which is incontact with the anode, and on the cathode side by a cation-exchangemembrane from the cathode compartment which is in contact with thecathode. These two last-mentioned compartments are also referred to asconcentrate compartments. The undesired sulphate and orthophosphite ionsin the deposition bath are transported into the anode compartment andthe undesired sodium ions, which come from the sodium hypophosphiteused, are transferred into the cathode compartment. In laboratory tests,however, it has emerged that, in addition to the undesired sulphate,orthophosphite and sodium ions, the bath constituents important for thedeposition process, namely the nickel and hypophosphite ions and theorganic complexing agents (mostly carboxylic acids or anions thereof),are transported into the concentrate compartments.

In DE 43 10 366 C1, a method of regenerating electrolessnickel/phosphorus baths by electrodialysis is described. Thenickel/phosphorus bath to be regenerated is to this end led through acompartment in an electrodialysis cell which is separated from theadjacent compartments both on the cathode side and on the anode side byrespectively one anion-exchange membrane (diluate compartment). Throughthe application of an electrical field, ortho- and hypophosphite ionsare transferred into the concentrate compartment lying on the anode sideof the diluate compartment. This solution is then transported into thecathode compartment which is in contact with the cathode. Hypophosphitecan by transference pass from there into the diluate compartment again,whilst orthophosphite is reduced to hypophosphite at the cathode and thehypophosphite produced is then also to be transferred into the diluatecompartment. However, it has emerged in tests that this reductionreaction does not in fact take place. Furthermore the parallelconnection of a large number of the quoted cells is proposed. Even withthis cell, the disadvantage is not overcome which is inherent in themethod described by Y. Kuboi and R. Takeshita. Moreover, sulphate andsodium ions also accumulate in this solution.

In U.S. Pat. No. 5,419,821, too, an electrodialytic method ofregenerating electroless metallisation baths is described. In a similarmanner to DE 43 10 366 C1, hypophosphite and orthophosphite aretransferred via an anion-exchange membrane into a concentratecompartment on the anode side and thus separated. In this case, too, theconcentrate solution on the anode side is transferred into the cathodecompartment, such that hypophosphite can from there reach the diluatecompartment again. orthophosphite is precipitated through the additionof magnesium or calcium salts to the solution which flows through thiscompartment, and in this way removed from the overall process. What isdisadvantageous, however, is that disturbing sodium and sulphate ionscannot be removed from the nickel bath solution.

In order to overcome the disadvantages of the methods described above, amethod for the electrodialytic regeneration of electrolessnickel/phosphorus baths was proposed in EP 0 787 829 A1, in which themethod is used in two different variants. In each of the variants, thismethod is operated discontinuously. The one variant represents atwo-stage method in which the spent deposition solution is first ledinto the diluate compartment of an electrodialysis cell, which isdelimited from two concentrate compartments by an anion-exchangemembrane on the side facing the anode and by a monoselectivecation-exchange membrane on the side facing the cathode. Monoselectiveion-exchange membranes differ from normal ion-exchange membranes in thatthey do not allow singly charged ions to pass, nor even ions which aremultiply charged. In the first stage of the method, sodium,hypophosphite, orthophosphite, sulphate and carboxylic acid ions aretransferred to the adjacent compartments, whilst nickel ions remain inthe diluate compartment. Then the respective solutions are led into asecond electrodialysis cell in which the concentrate compartment isdisposed between two diluate compartments and separated from the latteron the anode side by a monoselective anion-exchange membrane and on thecathode side by a cation-exchange membrane. In this case, thehypophosphite and carboxylic acid anions and the sodium cations aretransferred into the diluate compartment again, but not theorthophosphite and sulphate ions. From the balance, therefore, theorthophosphite and sulphate ions are removed but not the sodium ions.Since the charge balance is guaranteed in each individual method step,the total amount of the orthophosphite and sulphate ions cannot beremoved since the proportion of anionic counterions corresponding to thesodium ions remaining in the diluate compartment must also remain in thediluate compartment. Thus the efficacy of the separation is considerablyimpaired.

In the second variant, which is designed as a single-stage method, thebath solution is placed in the cathode compartment of an electrodialysiscell comprising three electrolyte compartments, the central compartmentbeing separated from the other compartments on the anode side by amonoselective anion-exchange membrane and on the cathode side by amonoselective cation-exchange membrane. The solution contained in theanode compartment is led into the cathode compartment. The bath solutionis first led into the cathode compartment. Hypophosphite andorthophosphite ions are supposed to be transferred into the centralcompartment. However, this appears impossible since a cation-exchangemembrane is disposed between the two compartments. For this reason, itis not clear how the method can be realised.

The main problem of the known devices and methods accordingly consistsin guaranteeing as effective and complete removal as possible ofdisturbing ions from the nickel/phosphorus deposition solution. Thesesubstances are in particular sodium, orthophosphite and sulphate ions.Moreover the method should be able to be carried out as continuously aspossible during the operation of the bath and only require one methodstep, in order to minimise the outlay. The problem underlying thepresent invention is, therefore, to avoid these disadvantages.

Accordingly, the invention relates to a method and a device forregenerating by electrodialysis electroless metal deposition bathscontaining hypophosphite ions as the reducing agent, especially bathsfor depositing nickel/phosphorus layers, and proceeds from the fact thatthe bath solution is led through diluate compartments in a firstelectrodialysis unit having cathodes and anodes, which compartments areseparated from concentrate compartments in the electrodialysis unit onthe cathode side by monoselective cation-exchange membranes and on theanode side by anion-exchange membranes. The bath solution is also ledsimultaneously through diluate compartments in a second electrodialysisunit, connected hydraulically in parallel to the first unit and havingcathodes and anodes, which compartments are separated from concentratecompartments in the second electrodialysis unit on the cathode side bymonoselective anion-exchange membranes and on the anode side byanion-exchange membranes. In both the electrodialysis units, the diluatecompartments and the concentrate compartments are disposed respectivelyalternately to one another.

In the simplest embodiment of the invention, the device has thefollowing equipment features:

a. a first electrodialysis unit, containing two concentrate compartmentsand a diluate compartment disposed between same as electrolytecompartments, the diluate compartment being separated on the cathodeside from the one concentrate compartment by a monoselectivecation-exchange membrane and on the anode side from the otherconcentrate compartment by an anion-exchange membrane,

b. a second electrodialysis unit, containing two diluate compartmentsand a concentrate compartment disposed between same as electrolytecompartments, the concentrate compartment being separated on the cathodeside from the one diluate compartment by an anion-exchange membrane andon the anode side from the other diluate compartment by a monoselectiveanion-exchange membrane, furthermore

. in each electrodialysis unit at least one cathode and at least oneanode and

d. a power supply for the cathodes and the anodes.

The spent bath solution, which as well as the valuable substances in thebath, i.e. hypophosphite, carboxylic acid and nickel ions, also containsdisturbing accompanying substances, namely, for example, orthophosphite,sulphate and sodium ions, is led simultaneously into all the diluatecompartments of the two electrodialysis units. Through transference, inthe first electrodialysis unit all the anions are transferred from thediluate compartment into the concentrate compartments disposed on theanode side of same, and the sodium ions into the concentratecompartments disposed on the cathode side of same, whilst nickel ionsremain in the diluate compartment. In the second electrodialysis unit,only the monovalent anions, namely hypophosphite and carboxylic acidions, are transferred from the concentrate compartments into the diluatecompartments on the anode side, whilst in this case the cationscontained in the concentrate compartment and the divalent anions, namelyorthophosphite and sulphate ions, remain in this compartment.

Since in the first electrodialysis unit a monoselective cation-exchangemembrane is used in the diluate compartment on the cathode side, sodiumions are transferred selectively from the diluate compartment into theconcentrate compartment. Nickel ions cannot escape from the diluatecompartment because of the special arrangement of the membranes.Moreover, through an anion-exchange membrane being used in bothelectrodialysis units in the diluate compartment on the anode side,hypophosphite is admittedly transferred from the diluate compartmentinto the concentrate compartment, but also orthophosphite and sulphate.The loss of hypophosphite and carboxylic acid ions from the diluatecompartment is selectively compensated for again, by a monoselectiveanion-exchange membrane being disposed in the second electrodialysisunit in the concentrate compartment on the anode side, such that theseions are selectively transferred from the concentrate compartment to thediluate compartment.

Thus, in the balance, during continuous circulation of the solutionthrough the two electrodialysis units, only the sodium, orthophosphiteand sulphate ions are removed from the spent solution, whilst thevaluable substances remain in the solution. With the method according tothe invention and the device, the optimal efficiency of the separationof disturbing bath constituents and thus the solution of the problemunderlying the invention is consequently achieved.

By both electrodialysis units being operated hydraulically in paralleland not in a sequential method, electroneutrality must be guaranteed inrespect of the ion transport only within the total arrangement. Thismeans that only in respect of the total arrangement must the amount ofanionic substances, which pass the membranes in the anodic direction, beequal to the amount of cationic substances which pass the membranes inthe cathodic direction. The bath solution circulates continuously againand again through both electrodialysis units, such that the disturbingsubstances, which are at first only partially separated, are graduallycompletely separated. For this reason, disadvantageous effects such asthose connected with the two-stage method of EP 0 787 829 A1are notobserved.

In order to achieve, in particular, continuous operation of theelectrodialytic method, a concentrate solution is led simultaneouslythrough the concentrate compartments. This concentrate solution containsthe disturbing substances removed substantially through enrichment fromthe spent bath solution. So that the concentration of these disturbingsubstances does not rise above a critical value, the concentratesolution is diluted continuously or at least from time to time(intermittently). Moreover, sodium hydroxide can be added to thissolution. This addition renders possible an effective separation of theorthophosphite ions from the hypophosphite ions, by an optimal pH valueof the concentrate solution being set above roughly 8.5 (forming HPO₃ ²⁻from H₂PO₃ ⁻)

What is guaranteed in this way is that the disturbing bath constituentscan be continuously removed from the spent solution. Otherwise, thesesubstances would accumulate in the concentrate solution above a criticalvalue and lead to a reduction in the separation effect, since thedisturbing substances could in these circumstances only be inadequatelytransferred into the concentrate solution.

In order to exploit the advantages of the electrodialytic method,preferably in the first electrodialysis unit respectively at least twodiluate compartments and at least three concentrate compartments aredisposed alternately to one another, and in the second electrodialysisunit respectively at least two concentrate compartments and at leastthree diluate compartments. In this way, with predetermined dimensionsof the ion-exchange membranes, a sufficiently large exchange area forthe spent bath solution is made available in the membranes. The largerthis exchange surface is, the faster and more effectively theregeneration of the bath can progress also. Therefore, in an optimalconfiguration for the regeneration arrangement, a large number ofdiluate compartments and concentrate compartments in the firstelectrodialysis unit and a large number of diluate compartments andconcentrate compartments in the second electrodialysis unit are disposedin respectively alternating sequences to one another. In this way, twostacks of electrolytic cells are created through which the diluatesolution is led through the diluate compartments and the concentratesolution through the concentrate compartments. Basically, the twoelectrodialysis stacks do not have to have the same number ofelectrolyte compartments. For example, it can be advantageous to providea larger number of diluate compartments and concentrate compartments inthe first electrodialysis unit than in the second electrodialysis unit.

The special arrangement of the ion-exchange membranes results in theconcentrate compartments in the first electrodialysis unit beingdelimited on the cathode side by anion-exchange membranes and on theanode side by monoselective cation-exchange membranes. The anode and thecathode are disposed on the end faces of the electrodialysis stack. Theelectrolyte compartments in contact with the cathode and the anode are,differently from the given sequence of membranes which separate therespective compartments from one another, are separated from theadjacent electrolyte compartments by cation-exchange membranes. In theseouter electrolyte compartments is to be found an electrochemically inertconducting salt solution, for example a sodium sulphate solution. Thisguarantees that no undesired electrode reactions take place in thesecompartments, which would lead to destruction of the electrodes or tothe formation of undesired reaction products on the electrodes.

In the same manner, the concentrate compartments in the secondelectrodialysis unit are delimited on the cathode side by anion-exchangemembranes and on the anode side by monoselective anion-exchangemembranes. In this case too, an anode or respectively a cathode isdisposed on the end faces of this second electrodialysis stack. Theelectrolyte compartments in contact with the cathode and the anode,differently from the given sequence of membranes which demarcate thediluate and concentrate compartments from one another, are separatedfrom the adjacent electrolyte compartments by cation-exchange membranes.In this second case, too, correspondingly inert solutions are to befound in the cathode compartment and the anode compartment, such that noundesired electrode reactions can take place.

The surface ratio of the normal anion-exchange membranes to themonoselective anion-exchange membranes in both electrodialysis stacksand the pH value of the solution led through the concentratecompartments (preferably at least 8.5) determine the degree of loss ofanionic valuable substances, i.e. of hypophosphite and carboxylic acidanions.

In a preferred embodiment, the first electrodialysis unit and the secondelectrodialysis unit are combined in a common electrodialysis stack anddisposed in such a manner that a cathode is disposed on only one endface of the common electrodialysis stack, and an anode on the other. Tothis end, the respective stacks are not electrically insulated from oneanother. Rather, for this purpose, an anion-exchange membrane isarranged on the boundary surfaces between the stacks to delimit the endconcentrate compartment on the cathode side of the first electrodialysisunit from the end diluate compartment on the anode side of the secondelectrodialysis unit. In this case, the corresponding cathodecompartment provided on the end electrolyte compartments is dispensedwith, as are the corresponding anode compartment and the associatedelectrodes. In this case, therefore, only one cathode compartment andone anode compartment are provided on the end faces of the stack, aswell as one cathode and one anode there.

In a further preferred alternative embodiment of the invention, thefirst electrodialysis unit and the second electrodialysis unit are againcombined in a common electrodialysis stack; in this case, however, thesequence of the individual electrolyte compartments is so selected thatthe electrolyte compartments of the one electrodialysis unit, which arealigned towards the cathode, are aligned towards the respectively otherelectrodialysis stack. Between the two electrodialysis units aredisposed a common cathode, and respectively one anode on the two endfaces of the common electrodialysis stack. This combination has theadvantage that only one stack has to be manufactured. In this case, twopower supplies are provided, namely a power supply for the cathode andthe one anode and a further power supply for the cathode and the otheranode. The electric circuits of the two electrodialysis units can, ofcourse, also be connected in parallel, such that again one power supplyis sufficient.

In an alternative embodiment to the above, the reverse sequence of theindividual electrolyte compartments is chosen. In this case, theelectrolyte compartments of the one electrodialysis unit, which arealigned towards the anode, are aligned towards the respectively otherstack of electrolytic cells. Between the two electrodialysis units isdisposed a common anode and on the two end faces of the commonelectrodialysis stack respectively one cathode.

In a further preferred embodiment according to the invention, the bathsolution of the deposition bath is led in a first circuit via a diluatecontainer. For this purpose, solution guiding means (pipelines, hoses)are provided between the container in which the deposition bath islocated and the diluate container. For example, the deposition solutionis circulated by suitable pumps continuously from the bath containerinto the diluate container, and from there back into the bath container.The solution contained in the diluate container is led in a secondcircuit through the diluate compartments in the first and the secondelectrodialysis unit, and from there back again. The solution istherefore transported via the diluate container into the diluatecompartments of the electrodialysis units and not directly from the bathcontainer into the electrodialysis units. By this means, greaterflexibility of the plant is achieved, since the volume flow (circulatingvolume of liquid per time unit) can be adjusted in the two circuitsindependently of one another.

In a particularly preferred embodiment, the volume flow in the secondcircuit is set higher than the volume flow in the first circuit by atleast one order of magnitude. The volume flow in the first circuit ispreferably even at the most 1% of the volume flow in the second circuit.What is thereby achieved is that only a small volume flow of the bathsolution which is regularly heated up to a high temperature has to becooled, so that the heat-sensitive ion-exchange membranes andinstallation parts in the electrodialysis units are not destroyed, andsubsequently heated up again. In this way, low heat losses are achievedsuch that a heat exchanger can possibly be dispensed with. Forcontinuous removal of disturbing substances from the depositionsolution, a relatively large liquid volume flow is continually ledthrough the diluate compartments. The liquid is cooled during transferinto the diluate container. Special heat-exchangers are not required forthis. Since only a small volume flow is conveyed into the diluatecontainer, only a little heat has to be taken from the bath solution andadded again during the return. Thus, the heat loss is low.

The diluate container can, moreover, be used to track the bathcomponents used up during the metal deposition, namely nickel andhypophosphite ions. Through metering corresponding substances, forexample, nickel sulphate and sodium hypophosphite, into the diluatecontainer, these substances can be completely mixed with the depositionsolution flowing through, before the solution enriched with thesesubstances enters the bath container again. If these substances areadded directly to the bath container, there is the danger that nickel isdeposited in metallic form on container fittings or walls, since withthe addition of the salts, locally increased concentrations of thesesubstances are formed.

In addition, a concentrate container can be provided from which theconcentrate solution is led into the concentrate compartments in theelectrodialysis stack and from there back again to the concentratecontainer. In order to maintain a suitable concentration of theconstituents of the concentrate solution, there is preferably disposedin the concentrate container a water supply, with which dilution of thesolution is possible. Through the passage of the disturbing substancesout of the diluate into the concentrate, these substances accumulatecontinuously in the concentrate, such that dilution becomes necessary.The supply of water is controlled, for example, by the electricalconductivity of the concentrate solution. The NaOH solution is alsometered into this container.

The monoselective ion-exchange membranes mentioned here are thoseion-exchange membranes which only allow ions with a single charge topass, monoselective cation-exchange membranes, i.e. sodium and hydronium(H₃O*) ions, and monoselective anion-exchange membranes, for examplehypophosphite, hydroxide and carboxylic acid anions, whilst thesemembranes are substantially impermeable to multiply charged ions, i.e.nickel, sulphate and orthophosphite ions. If reference is only made toanion- or cation-exchange membranes, without referring to monoselectiveproperties, these are those ion-exchange membranes which have noselectivity in respect of the number of charges of the ions passing.

BRIEF DESCRIPTION OF DRAWINGS

The invention is explained in greater detail below with the aid offigures. These show in detail:

FIG. 1: a schematic representation of the partial processes in the firstand in the second electrodialysis unit;

FIG. 2: a schematic representation of a first embodiment of the deviceaccording to the invention;

FIG. 3: a schematic representation of a second embodiment of the deviceaccording to the invention.

In FIG. 1, the basic structure of the electrodialysis units in thesimplest embodiment is represented schematically. In both cases, anodesAn and cathodes Ka are contained in the corresponding anode compartmentsAR1, AR2 or respectively the corresponding cathode compartments KR1,KR2. In these compartments is located exchangeable electrolyte solution,preferably a sodium sulphate solution.

The anode or cathode compartments are separated from the adjacentelectrolyte compartments by cation-exchange membranes K. Membranes ofthis type, just as the remaining ion-exchange membranes used, are freelyavailable, for example from the company DuPont de Nemours, U.S.A.

The diluate solution flows through all the diluate compartments Di andthe concentrate solution through all the concentrate compartments Ko.This is indicated schematically by the arrows.

In the electrodialysis unit E1, which is represented schematically inthe upper portion of FIG. 1, a first concentrate compartment Ko1 acommunicates with the anode compartment AR1. The two compartments areseparated from one another by a cation-exchange membrane K. Through theconcentrate compartment Ko1 a flows the concentrate solution, preferablya slightly alkaline solution which, during operation, contains thesubstances which are taken up from the diluate solution (for exampleorthophosphite, sulphate, sodium ions). This first concentratecompartment is delimited on the cathode side by an anion-exchangemembrane A. Towards the cathode, a diluate compartment Di1 a, throughwhich the diluate solution flows, communicates with the concentratecompartment Ko1 a. On the cathode side, a concentrate compartment Ko1 b,through which the concentrate solution flows, communicates in turn withthe diluate compartment. Compartments Di1 a and Ko1 b are separated fromone another by a monoselective cation-exchange membrane KS. Theconcentrate compartment Ko1 b is divided from the adjacent cathodecompartment KR1 by a cation-exchange membrane K.

Sodium ions contained in the concentrate compartment Ko1 a are nottransferred into the diluate compartment Di1 a. In the diluate solutionare found, in the case of a typical nickel/phosphorus deposition bath,nickel, sodium, hypophosphite (H₂PO₂ ⁻), orthophosphite (HPO₃ ²⁻),sulphate and carboxylic acid (RCOO⁻) ions. Of the types of ions locatedin the diluate compartment Di1 a, all the anions, i.e. hypophosphite,orthophosphite, sulphate and carboxylic acid anions are transferredthrough the anion-exchange membrane A into the concentrate compartmentKo1 a, and of the cations, the singly charged sodium and hydronium ionsare transferred through the monoselective cation-exchange membrane KSinto the concentrate compartment Ko1 b. On the other hand, thedouble-charged nickel ions are not transferred into the concentratecompartment Ko1 b but remain in the diluate compartment. Hydroxide ions,possibly contained in the concentrate compartment Ko1 b in a lowconcentration, cannot pass into the diluate compartment. The same isalso true for the hypophosphite, orthophosphite, sulphate and carboxylicacid ions.

In the overall balance of the electrodialysis unit E1, therefore, allthe anions are transferred into the concentrate compartment, whilst ofthe cations only the sodium ions and the hydronium ions pass into theconcentrate compartment, but not the nickel ions.

In the electrodialysis unit E2, which is represented schematically inthe lower portion of FIG. 1, a first diluate compartment Di2 bcommunicates with the anode compartment AR2. The anode compartment isdelimited on the cathode side by a cation-exchange membrane K. Thediluate solution flows through this diluate compartment. The diluatecompartment is delimited on the cathode side by a monoselectiveanion-exchange membrane AS. On the cathode side, there adjoins aconcentrate compartment Ko2 a, through which the concentrate solutionflows. This compartment is divided by an anion-exchange membrane A froman adjacent second diluate compartment Di2 a, through which the diluatesolution flows. This second diluate compartment Di2 a is divided on thecathode side from the adjoining cathode compartment KR2 by means of acation-exchange membrane K.

Cations cannot pass from the first diluate compartment Di2 b into theadjoining concentrate compartment Ko2 a, since the two compartments areseparated from one another by a monoselective anion-exchange membraneAS.

Equally, sodium ions contained in the concentrate compartment cannotpass into the second diluate compartment Di2 a, since in this case thetransfer of sodium ions is opposed by an anion-exchange membrane. Anionscontained in the second diluate compartment Di2 a, namely hypophosphite,orthophosphite, sulphate, carboxylic acid and hydroxide ions, aretransferred into the central concentrate compartment Ko2 a. Of theanions which have reached the concentrate compartment, only the singlycharged anions can pass through the monoselective anion-exchangemembrane AS into the diluate compartment Di2 b, namely hypophosphite,carboxylic acid and hydroxide ions.

In the overall balance of the partial processes running in thiselectrodialysis unit, the disturbing bath constituents are thusselectively transferred into the concentrate compartment, whilst thevaluable substances are returned again to the diluate solution afterpassing the concentrate compartment.

The electrodialysis unit according to the invention comprises twoelectrodialysis stacks E1 and E2, as shown in FIG. 2. These are shown inthe detail in the lower portion of FIG. 2, separately enlarged as abasic unit. The two stacks are combined into a common stack. Theelectrodes are attached to the end faces of the common stack, in FIG. 2on the left-hand side an anode An and on the right-hand side the cathodeKa. As the anode is used, for example, a stainless steel plate ortitanium coated with noble metal mixed oxides or platinum-plated. Aplate of the same material can be used for the cathode. The individualelectrodialysis cells within the stack comprise respectively speciallyshaped frames which leave the diluate compartments Di or concentratecompartments Ko free and have ducts in order to permit a guided flow ofthe diluate solution, on the one hand, and of the concentrate solutionon the other hand, through the individual compartments. The ducts arehere so formed that the liquid coming from the diluate container V_(D)can enter simultaneously all the diluate compartments Di and the liquidcoming from the concentrate container V_(K) can enter simultaneously allthe concentrate compartments Ko.

Furthermore, seals are contained in the stack in order to prevent anyescape of liquid from the stack or passing of liquid from onecompartment to an adjacent compartment. On the end surfaces are providedforce-absorbing plates, made of steel for example. The whole stack isscrewed by means of bolts, which extend through the entire stack, ortensioned hydraulically.

The whole stack has, moreover, the ion-exchange membranes which arerequired for separating the types of ion and which separate theindividual compartments from one another. The electrodialysis unit E1comprises diluate compartments Di1 a, Di1 b, Di1 c, . . . , Di1 x, andconcentrate compartments Ko1 a, Ko1 b, Ko1 c, . . . , Ko1 x, disposedalternating with one another. Towards the cathode side, the diluatecompartments are separated from the concentrate compartments bymonoselective cation-exchange membranes KS and towards the anode side byanion-exchange membranes A. The anode An is in direct contact with theouter compartment of the electrodialysis unit E1 on the anode side. Thisis the anode compartment here. The anode compartment is separated fromthe adjacent concentrate compartment Ko1 a by a cation-exchange membraneK.

At the outer concentrate compartment Ko1 x on the cathode side,electrodialysis unit E1 is connected with electrodialysis unit E2. Theconnection point is provided by an anion-exchange membrane A. On thecathode side is located, adjacent to this anion-exchange membrane, adiluate compartment Di2 x of unit E2. In this electrodialysis unit E2,the diluate compartments Di2 x, . . . , Di2 c, Di2 b, Di2 a and theconcentrate compartments Ko2 x, . . . , Ko2 c, Ko2 b, Ko2 a alternatewith one another. For example, two diluate compartments Di1 and threeconcentrate compartments Ko1 can be combined in electrodialysis unit E1and three diluate compartments Di2 and two concentrate compartments Ko2can be combined in electrodialysis unit E2.

Each diluate compartment Di2 is separated from the adjacent concentratecompartments Ko2 by an anion-exchange membrane A towards the anode sideand by a monoselective anion-exchange membrane AS towards the cathodeside.

The cathode Ka is in direct contact with the outer compartment on thecathode side of electrodialysis unit E2. This is the cathodecompartment. The cathode compartment is separated from the adjacentdiluate compartment Di2 a by a cation-exchange membrane.

The anode An and the cathode Ka are connected with a rectifying powersupply S.

The bath solution is pumped, coming from the bath container B, via apipeline R₁, into the diluate container V_(D), for example with a volumeflow of 20 l/h. The solution in the container V_(D) is led via a furtherpipeline R₂ back into the container B. In the diluate container V_(D),the nickel/phosphorus deposition solution, entering at a temperature of,for example, 90° C., cools down to a temperature of 40° C., for example.

From the diluate container, the deposition solution is conveyed by meansof a pump P_(D) via a pipeline R₃ into all the diluate compartments Di1and Di2 of the electrodialysis units E1 and E2. The volume flow is forexample 7 m³/h. After the solution has passed through the diluatecompartments, it returns via pipeline R₄ to the diluate container.

A concentrate solution flows through the concentrate compartments Ko1and Ko2 of the two electrodialysis units. The concentrate solution islocated in the concentrate container V_(K). The solution is conveyed bymeans of a pump P_(K) via pipeline R₅ simultaneously into all theconcentrate compartments. After the solution has passed through thesecompartments, it returns to the concentrate container via pipeline R₆.Since the disturbing substances located in the deposition solution, suchas orthophosphite, sulphate and sodium ions, constantly accumulate inthe concentrate solution, the latter must be continuously diluted inorder to prevent any inhibition of the transfer of these types of ionthrough the ion-exchange membranes. To this end, water is added to theconcentrate container continuously or intermittently.

In order, furthermore, to set an optimal pH value for the selectivetransfer of orthophosphite ions in the concentrate solution, the pHvalue of the concentrate solution is set at values above 8.5 by addingsodium hydroxide to the solution. This hydroxide must be continuouslyadded since hydroxide ions are used up by conversion of HPO₃ ²⁻ intoH₂PO³⁻ and are thus lost from the concentrate solution.

In a further embodiment (FIG. 3) the electrodialysis units E1 and E2,shown as per FIG. 2, are used. The two units are also combined in acommon stack, however in the manner that the cathode sides of the twounits adjoin one another and a cathode Ka is disposed between the twoindividual stacks. In this case, the sequence of the anion-exchangemembranes in electrodialysis unit E2 is reversed.

In this case, too, cation-exchange membranes are provided between thecathode compartments and the adjoining electrolyte compartments on theone hand, and between the anode compartments and the adjoiningelectrolyte compartments on the other hand.

To supply power, again a rectifier is used which supplies bothelectrodialysis stacks simultaneously, by the two stacks beingelectrically connected to one another in parallel. The electricalcircuit through the cathode Ka and the anode An₁ is connected inparallel with the electrical circuit through the cathode Ka and theanode An₂.

The remaining elements of the device are identical with those of thefirst embodiment.

An example is quoted below to further clarify the invention:

Nickel/phosphorus alloy layers were deposited from a suitable bath ontosteel plates. The nickel/phosphorus bath initially had the followingcomposition:

Na⁺ (from NaH₂PO₂) 6.5 g/l Ni²⁺ (from NiSO₄) 7.0 g/l HPO₃ ²⁻ (formed byoxidation from 0 g/l hypophosphite) H₂PO₂ ⁻ (from NaH₂PO₂) 18 g/l SO₄ ²⁻(from NiSO₄) 12 g/l Lactic acid 30 g/l Propionic acid 5 g/l Pb²⁺ fromPb(NO₃)₂ 2 mg/l With the following characteristics: pH value 4.6Temperature 85° C. Deposition speed 12 to 14 μm/h

After ageing of the bath to 5.6 MTO, the bath was exhausted and had thefollowing concentrations or parameters:

Na⁺ 46 g/l Ni²⁺ 6 g/l HPO₃ ²⁻ 134 g/l H₂PO₂ ⁻ 18 g/l SO₄ ²⁻ 66 g/l pHvalue 5.0 Temperature 90° C. Deposition speed 5 μm/h

After the ageing of the bath, the quality of the nickel/phosphoruscoating had sunk to a limit which was no longer acceptable. The baththerefore had to be thrown away.

In a second test, a bath was operated with the above-quoted initialcomposition and continuously regenerated using the device represented inFIG. 2. The conditions are quoted below:

Bath container volume 1 m³ Bath load (metal surface to be coated 10m²/m³ per bath volume) Volume flow from bath to diluate 30 l/h containerVolume flow from diluate container 6000 l/h to electrodialysis unit Heatlosses 0.8 kW Electrical power consumption 4.2 kW

Through the comparatively low volume flow from the bath to the diluatecontainer, expensive and high-loss heat exchange for cooling the bathand later re-heating of the returned solution was avoided. It was onlynecessary to lead away the electrical power used for the electrodialysisin order not to exceed the maximum admissible temperature in theelectrodialysis stack. For this cooling, rinsing water of a hot waterrinse was used which was needed in the treatment of metal surfaces to benickel-plated and had to be heated up anyway.

The concentrations of the individual bath constituents and the bathparameters could here be kept constantly at the following values:

Na⁺ 24 g/l Ni²⁺ 7.0 g/l HPO₃ ²⁻ 60 g/l H₂PO₂ ⁻ 18 g/l SO₄ ²⁻ 36 g/l pHvalue 4.7 Temperature 88° C. Deposition speed 12 μm/h

The composition of the bath obtained corresponded, proceeding from thenewly started bath, to a deposition bath with an age of roughly 2 to 3MTO.

What is claimed is:
 1. Method for regenerating by electrodialysis anelectroless metal deposition bath, containing hypophosphite ions as thereducing agent, in which the liquid of the bath is led through diluatecompartments (Di1 a, Di1 b, . . . , Di1 x) in a first electrodialysisunit (E1) having cathodes (Ka) and anodes (An), which compartments areseparated from concentrate compartments (Ko1 a, Ko1 b, . . . , Ko1 x) inthe electrodialysis unit (E1) on the cathode side by monoselectivecation-exchange membranes (KS) and on the anode side by anion-exchangemembranes (A), the diluate compartments (Di1 a, Di1 b, . . . , Di1 x)and the concentrate compartments (Ko1 a, Ko1 b, . . . , Ko1 x) beingdisposed alternately to one another, characterised in that the bathsolution is led simultaneously through diluate compartments (Di2 a, Di2b, . . . , Di2 x) in a second electrodialysis unit (E2) having cathodes(Ka) and anodes (An, An₂), which compartments are separated fromconcentrate compartments (Ko2 a, Ko2 b, . . . , Ko2 x) in the secondelectrodialysis unit (E2) on the cathode side by monoselectiveanion-exchange membranes (AS) and on the anode side by anion-exchangemembranes (A), the diluate compartments (Di2 a, Di2 b, . . . , Di2 x)and the concentrate compartments (Ko2 a, Ko2 b, . . . , Ko2 x) in thesecond electrodialysis unit (E2) being disposed alternately to oneanother.
 2. Method according to claim 1, characterised in that aconcentrate solution is led simultaneously through the concentratecompartments (Ko2 a, Ko2 b, . . . , Ko2 x).
 3. Method according to oneof the preceding claims, characterised in that the first electrodialysisunit (E1) and the second electrodialysis unit (E2) are combined in acommon electrodialysis stack and so disposed that a cathode (Ka) isdisposed only on one end face of the common electrodialysis stack, andan anode (An) on the other.
 4. Method according to one of claims 1 and2, characterised in that the first electrodialysis unit (E1) and thesecond electrodialysis unit (E2) are combined in a commonelectrodialysis stack and so disposed, a. that between the twoelectrodialysis units (E1, E2), a common cathode (Ka) is disposed andrespectively one anode (An₁, An₂) on the two end faces of the commonelectrodialysis stack (E1, E2), or b. that between the twoelectrodialysis units (E1, E2) a common anode (An) is disposed andrespectively one cathode (Ka) on the two end faces of the commonelectrodialysis stack (E1, E2).
 5. Method according to one of thepreceding claims 1 and 2, characterised in that the bath solution of thedeposition bath is led in a first circuit via a diluate container(V_(D)) and the liquid contained in the diluate container (V_(D)) is ledin a second circuit through the diluate compartments (Di1 a, Di1 b, . .. , Di1 x, Di2 a, Di2 b, . . . , Di2 x) in the first and the secondelectrodialysis unit (E1, E2), the volume flow in the second circuitbeing greater than the volume flow in the first circuit by at least oneorder of magnitude, preferably by at least two orders of magnitude. 6.Method for regenerating by electrodialysis an electroless nickeldeposition bath containing hypophosphite ions as the reducing agent,according to one of the preceding claims 1 and
 2. 7. Device forregenerating by electrodialysis an electroless metal deposition bathcontaining hypophosphite ions as the reducing agent, the devicecontaining a. a first electrodialysis unit (E1) containing twoconcentrate compartments (Ko1 a, Ko1 b) and a diluate compartment (Di1a), disposed between same, as electrolyte compartments, the diluatecompartment (Di1 a) being separated on the cathode side from the oneconcentrate compartment (Ko1 b) by a monoselective cation-exchangemembrane (KS) and being separated on the anode side from the otherconcentrate compartment (Ko1 a) by an anion-exchange membrane (A), b. inthe first electrodialysis unit (E1) at least one cathode (Ka) and atleast one anode (An) and c. a power supply (S) for the cathodes (Ka) andthe anodes (An, An₁), characterised by d. a second electrodialysis unit(E2) containing two diluate compartments (Di2 a, Di2 b) and aconcentrate compartment (Ko2 a), disposed between same, as electrolytecompartments, the concentrate compartment (Ko2 a) being separated on thecathode side from the one diluate compartment (Di2 a) by ananion-exchange membrane (A) and being separated on the anode side fromthe other diluate compartment (Di2 b) by a monoselective anion-exchangemembrane (AS), as well as at least one cathode (Ka) and at least oneanode (An, An₂) and a power supply (S) for the cathodes and the anodes.8. Device according to claim 7, characterised in that in the firstelectrodialysis unit (E1) respectively at least two diluate compartments(Di1 a, Di1 b) and at least three concentrate compartments (Ko1 a, Ko1b, Ko1 c) are disposed alternately to one another, and in the secondelectrodialysis unit (E2) respectively at least two concentratecompartments (Ko2 a, Ko2 b) and at least three diluate compartments (Di2a, Di2 b, Di2 c) are disposed alternately to one another.
 9. Deviceaccording to one of claims 7 and 8, characterised in that theconcentrate compartments (Ko1 a, Ko1 b, . . . , Ko1 x) in the firstelectrodialysis unit (E1) are delimited on the cathode side byanion-exchange membranes (A) and on the anode side by monoselectivecation-exchange membranes (KS), with the proviso that the electrolytecompartments (KR1, AR1) in contact with the cathodes (Ka) or the anodes(An, An₁) are separated from the adjacent electrolyte compartments bycation-exchange membranes (K).
 10. Device according to one of claims 7and 8, characterised in that the concentrate compartments (Ko2 a, Ko2 b,. . . , Ko2 x,) in the second electrodialysis unit (E2) are delimited onthe cathode side by anion-exchange membranes (A) and on the anode sideby monoselective anion-exchange membranes (AS), with the proviso thatthe electrolyte compartments (KR2, AR2) in contact with the cathodes(Ka) or the anodes (An, An₂) are separated from the adjoiningelectrolyte compartments by cation-exchange membranes (K).
 11. Deviceaccording to one of claims 7 and 8, characterised in that first liquidguiding means (R₁, R₂) are provided, by means of which the liquid of thebath may be led in a first circuit through a diluate container (V_(D)),and in addition second liquid guiding means (R₃, R₄), by means of whichthe liquid in the diluate container (V_(D)) may be led in a secondcircuit from the diluate container (V_(D)) through the diluatecompartments (Di1, Di2) in the first (E1) and in the second (E2)electrodialysis unit and from there back again.
 12. Device according toone of claims 7 and 8 characterised in that the first electrodialysisunit (E1) and the second electrodialysis unit (E2) are combined in acommon electrodialysis stack and so disposed, that a cathode (Ka) isdisposed only on one end face of the common electrodialysis stack, andan anode (An) on the other one. 13.Device according to one of claims 7and 8 characterised in that the first electrodialysis unit (E1) and thesecond electrodialysis unit (E2) are combined in a commonelectrodialysis stack and so disposed, a. that a common cathode (Ka) isdisposed between the two electrodialysis units and respectively oneanode (An₁, An₂) on the two end faces of the common electrodialysisstack, or b. that between the two electrodialysis units is disposed acommon anode (An) and respectively one cathode (Ka) on the two end facesof the common electrodialysis stack.